Electrical Wiring and Circuit Capacity

If you plan on wiring an outlet or light fixture onto an existing electrical circuit you will have to determine if the circuit has enough capacity to handle the additional load.

Start by adding up the total wattage that is currently being consumed on the circuit you want to add to.  If you don’t know what is powered by the circuit in question, switch it off at the breaker box and then turn lights and electrical appliances on and off throughout the house.  If the item doesn’t turn on then you know it is part of the circuit.

Since you probably don’t have the operator’s manual for every electrical appliances you ever bought, it may be difficult to determine how much wattage each item consumes. For that reason, you can use this handy wattage calculator to help find your total.  Be sure to include wattage for appliances that are occasionally plugged into the circuit too, such as a vacuum, or power tool. And, don’t forget to include the wattage for the new fixture you’re adding to the circuit.

Once you have your total wattage figure, check the circuit breaker for its rating.  A 15 amp circuit can handle up to 1800 watts and up to 2400 watts on a 20 amp circuit.  If the power needed is more than 80% of those numbers you may want to consider a different circuit or running a new circuit.  Reason being, some appliances have starting loads that are 3 times their running load.  What this means is, you plug your circular saw into the new outlet you just installed and when you start it up, your workshop lights go out.  That’s because you calculated your total wattage using the running load of each appliance.  The high start up load required by the circular saw put too much demand on the circuit which tripped the breaker.  For this reason you need that 20% buffer to account for the start up demand from certain appliances.

Finally, remember to run the correct wiring for your new outlet or light fixture, 14 gauge for a 15 amp circuit and 12 gauge for a 20 amp circuit.  Also be aware that by code, you can not add to the circuit that powers the kitchen counter top outlets.